[Speed] intel_pstate C0 bug on isolated CPUs with the performance governor

[Victor Stinner]

2016-09-23 12:19 GMT+02:00 Antoine Pitrou <solipsis at pitrou.net>:
> On Fri, 23 Sep 2016 11:44:12 +0200
> Victor Stinner <victor.stinner at gmail.com>
> wrote:
>> I guess that for some reasons, the CPU frequency was 1.6 GHz (min
>> frequency) even if I configured the CPU frequency governor to
>> performance.
>
> Does that mean that all "perf"-reported benchmark results that you
> reported from your machine are actually invalid?

I don't know why, but the bug only started to occur since one week.
The performance difference is so huge (2.0x factor) that it's easy to
spot it.

In fact, raw performance numbers don't matter. IMHO only comparison
between timings matter: the "...x faster" or "...x slower".

By the way, I now hesitate to try an advice that I read somewhere:
always use the minimum CPU frequency to run a benchmark. It avoids all
tricky speed changes of modern Intel CPUs. A CPU cannot run slower
than its minimum frequency. If the frequency is fixed, it cannot run
faster neither. So the CPU frequency issue should be fixed for all
known issues: Turbo Boost, speed change when close to the heat limit
(100°C), etc.

Victor